Toggle filesystem test mode.
Filesystem test mode, when turned on, prints out data corresponding to
all the device reads and what values are being sent to the low-level
routines. The format is `<partition-offset-sector,
byte-offset, byte-length>' for high-level reads inside a
partition, and `[disk-offset-sector]' for low-level sector
requests from the disk.
Filesystem test mode is turned off by any use of the install
(see install) or testload (see testload) commands.
